White stuff inside blue tetra

I have a blue tetra that I've had for a couple of months. When I first got it, 10 fish died. It seemed that only certain species died(cherry barbs, Kerri tetras, blue rams). Now, months later, the lone blue tetra has spots on the inside that I can see through the translucent skin. The fish acts normally and the survivors from the first dying off are just fine. If anyone can tell me what's wrong and how to treat it I will greatly appreciate it. Thanks!

Parameters:
Temp. 86f (treating it as if it's ich)
PH 6.6
Hardness 120
Nitrite 0ppm
Nitrate 30ppm


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